Ink jet recording is carried out in such a manner that minute ink droplets are allowed to jet utilizing various working principles, and to adhere onto a recording medium so that images and texts are recorded. Ink jet recording exhibits advantages of relatively high speed, low noise and the ease of multicolor formation.
Further, as a result of technical progress in recent years, the image quality of ink jet prints prepared by utilizing a dye ink have approached those prepared utilizing silver halide photography, and further, the price of such units has been reduced. As a result, ink jet printing has been increasingly employed.
Generally, dyes are soluble in solvents, and dye molecules are subjected to dying in either a molecular state or a cluster state. Accordingly, each dye molecule exists in a similar environment. Thus its absorption spectra have a narrow band and the resultant color is very pure as well as clear. In addition, since dyes do not exhibit particle patterns, light scattering nor light reflection, the dye ink can provide an ink jet image with high transparency as well as clear hue.
However, when dye molecules are decomposed due to photochemical reactions and the like, the number of dye molecules decreases. Since the color density is proportional to the number of dye molecules, a decrease in the number of dye molecules results in degradation of light fastness Ink jet recorded images utilizing a dye ink provide high image quality, but the image quality markedly degrades during storage. Accordingly, the present situation is such that, in terms of image quality stability, no ink jet technique has yet developed which exceeds silver halide photography in that regard.
While competing with dye ink, there is pigment ink in which pigments with good light fastness is employed as the colorant, as ink to be used to prepare images with desired light fastness. Pigment is easy to form particles as compared to dyes, and scatters light, resulting in images without transparency. Accordingly, pigment has the defect in that it is inferior in color reproduction to dyes.
In order to overcome such a defect, an attempt has been made to improve color reproduction by employing ink containing pigment particles with a small particle size, the pigment particles being obtained by dispersion processing. However, as primary pigment particles are made smaller, they become more difficult to be dispersed in a dispersion medium, which results in the problem in that viscosity of the resulting dispersion liquid increases. As a technique to solve such a problem, a technique is known in the art, which modifies the surface of the pigment particles with pigment derivatives or inorganic salts. Typically, there is a technique in which pigment particles are milled in a mill at high temperature for a long time or a technique in which pigment particles are dissolved in sulfuric acid and poured into water to form paste. Regarding quinacridone pigment, there is the proposal as disclosed in Japanese Patent Publication No. 54-135821 and in Japanese Patent O.P.I. Publication Nos. 5-230384, in which quinacridone is dissolved in DMSO together with pigment derivatives to form paste.
However, these techniques are those for preparing a dispersion liquid having a high content of polymers in an aqueous or non-aqueous dispersion medium such as a coating. They could not be applied to ink for ink jet recording to be required to have a low viscosity and an extremely low content of polymers. Their application to ink for ink jet recording necessitated an increased content of polymeric dispersants in the ink, which resulted in a problem in that jetting stability was deteriorated due to an increase in the ink viscosity, and therefore, they could not solve the fundamental problem as described above.